KR20150101469A - Coated electrode - Google Patents

Abstract

It is an object of the present invention to provide a coated arc welding electrode that combines excellent creep performance and various performance characteristics such as toughness, tempering embrittlement resistance, resistance to SR cracking, and welding workability. The coated arc welding electrode of the present invention is characterized in that, during the coating operation, the metal carbonate in an amount of 5 to 10 mass% in terms of CO ₂ , the alkali metal oxide in an amount of 0.4 to 2.0 mass% in terms of alkali metal, Si, Mn, Cr, Mo, V, and Nb per total mass of the coated arc welding electrode in the total amount of either or both of the core wire and the coated wire of the coated arc welding electrode, And the total sum of Cu, Ni, Ti, Al, Ta, Co, and W is 0.10 mass% or less as the inevitable impurities and the total sum of S, Sn, As, Sb, Pb, and Bi Is not more than 0.10 mass%.

Description

{COATED ELECTRODE}

The present invention relates to a coated arc welding electrode provided for welding a 2.25Cr-1Mo-V steel used in a pressure vessel in an energy plant, such as a petroleum refinery reactor.

2.25 Cr-1Mo steel has been conventionally used as a pressure vessel for a petroleum refinery reactor or the like. However, from the viewpoint of improving the operating efficiency, application of a 2.25Cr-1Mo-V forced reactor having a higher strength is progressing as a steel material capable of withstanding high temperature and high pressure of design temperature and pressure. Development of a coated arc welding electrode or a welding metal excellent in creep rupture performance and toughness as a coated arc welding electrode or a welding metal provided for welding of 2.25Cr-1Mo-V steel for use in such an oil refining reactor is underway.

For example, in Patent Document 1, a weld metal formed by covered arc welding is made to contain 0.20 to 0.70% by mass of V with respect to 2.00 to 3.25% by mass of Cr and to secure creep strength by adding Nb, Co and W . In Patent Document 2, the content of V is 0.1 to 1.0 mass% and Nb is 0.02 to 0.50 mass% with respect to 1.00 to 3.50 mass% of Cr, and the addition of Co, W, And toughness. Patent Document 3 discloses that the content of V is 0.01 to 0.60 mass% and W is 0.6 to 2.5 mass% with respect to 1.00 to 3.50 mass% of Cr per mass of the electrode, and the creep strength is ensured by adding Nb and Ta .

Japanese Unexamined Patent Application Publication No. 10-137975 Japanese Patent Application Laid-Open No. 2002-263883 Japanese Patent Application Laid-Open No. 2001-300768

However, the conventional techniques have the following problems.

As a welding material to be used for welding a steel material for use in an oil refining reactor, excellent creep performance and various performance characteristics such as toughness, tempering embrittlement resistance, SR SR resistance (which does not cause grain boundary cracking during stress relief annealing) And the workability of welding are both required.

As described above, application of a 2.25Cr-1Mo-V forced reactor which is higher in strength than 2.25Cr-1Mo steel is underway. However, even in the 2.25Cr-1Mo-V forced reactor, the operating conditions tend to be high temperature and high pressure, and conventional welding materials have a problem of not satisfying requirements of high temperature strength, particularly creep rupture performance.

In Patent Document 1, the PWHT condition after welding is low temperature and short time at 700 占 폚 占 26 hours, and the creep test condition is low temperature and low stress at 538 占 폚 / 207 MPa, . In Patent Document 2, the PWHT condition is 740 占 폚, the holding temperature is high, the holding time is short, and the creep test condition is 600 占 폚 / 140 MPa. In Patent Document 3, the PWHT condition is 690 占 폚 for 1 hour, the holding temperature is low temperature, the holding time is short, and the creep test condition is 600 占 폚 / 140 MPa.

As a prior art for raising the creep rupture strength of the Cr-Mo steel weld metal in this way, it has been proposed to use hard strengthening elements such as V, Nb, Ta and W, such as strong carbide forming elements and Co, A method of additionally adding Cr-Mo to the Cr-Mo system has been employed. However, this results in deterioration of impact toughness and degradation of crack resistance due to an increase in hardness.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and it is an object of the present invention to provide a coated arc welding electrode that satisfies both excellent creep performance and various performance toughness, tempering embrittlement resistance, SR resistance and welding workability.

As a result of intensive studies, the present inventors have found the following.

The inventors of the present invention have found that, in the formation of a 2.25Cr-1Mo-V steel weld metal having excellent creep strength and excellent toughness, tempering brittleness and SR resistance resistance, as means for improving creep performance, (1) Shape control, and (2) microstructure control.

The microstructure of 2.25Cr-1Mo-V steel and its deposited metal is bainite-based, and its creep deformation behavior is mainly due to the creep diffusion of dislocations and the formation of bainite blocks, The slip of the boundary is considered dominant. For the suppression of electrons, it is effective to disperse fine carbides, which inhibit the creep diffusion of dislocations, into crystal grains. On the other hand, in the latter suppression, dispersion and precipitation of carbides which inhibit slippage in the old? -Grain system and reduction of slip sites, that is, coarsening of microstructures, are effective.

In the realization of the above (1), it is effective to add alloying elements which preferentially form fine carbides in the crystal grains, and attention is paid to C and V in the present application. These alloying elements may be added either from the core wire constituting the coated arc welding electrode and from the coating agent, or may be added from both sides.

In the realization of the above (2), it is effective to add an alloying element which forms a carbide preferentially on the old? -Grain system to inhibit slipping and addition of an alloying element which lowers the hardenability, or reduction of an alloying element which increases hardenability In the present invention, attention was paid to Cr and Mn. Particularly, it has been found that by appropriately reducing Cr, precipitation of fine carbides into the crystal grains during heat treatment is promoted, and the effect of suppressing diffusion of creep potential by the pin stop effect of the fine carbides can be obtained. The method of adding these alloying elements is the same as the above.

In the prior art, as described herein, creep rupture strength is improved remarkably in a low range of Cr as a deposited metal for a high strength 2.25Cr-1Mo-V steel, that is, the creep rupture strength is improved by suppressing the alloy element to a low level No such knowledge is suggested at all.

It should be noted that any of the welding materials constituted in the prior art is required to meet the creep characteristics of PWHT conditions: 705 占 폚 占 32 hours, creep rupture test conditions: 540 占 폚 / 210 MPa, Can not be satisfied.

The coated arc welding electrode according to the present invention is characterized in that, during the coating, the amount of the metal carbonate is 5 to 10% by mass in terms of CO ₂ , 0.4 to 2.0% by mass in terms of alkali metal in the alkali metal oxide, 0.04 to 0.15 mass% of C, 1.0 to 5.0 mass% of Si, 1.0 to 5.0 mass% of the compound in terms of F, and the total amount of either or both of the core wire and the covering of the coated arc welding electrode, Mo: 0.5 to 1.0 mass%, V: 0.3 to 0.5 mass%, Nb: 0.02 to 0.06 mass%, B: 0.005 to 0.015 mass% 0.1 to 5 mass% of at least one of Cu, Ni, Ti, Al, Ta, Co, and W as an inevitable impurity; S, Sn, As , Sb, Pb and Bi is 0.10 mass% or less.

According to such a configuration, the coated arc electrode contains a predetermined amount of metal carbonate during the coating, the weld bead is protected, the hydrogen partial pressure in the arc atmosphere is lowered, and the amount of diffusible hydrogen in the deposited metal is reduced. In addition, by containing a predetermined amount of alkali metal oxide during coating, good welding workability is imparted to the coated arc welding electrode. Further, by containing a predetermined amount of the fluorine compound in the coating, the fluorine compound acts as the slag forming agent without deteriorating the bead shape.

In addition, the coated arc welding electrode is characterized in that it contains creep rupture performance, toughness, tempering brittleness characteristic, and internal SR fracture property of the weld metal, and weldability of the weld metal by welding a predetermined amount of predetermined components to either or both of the core wire Workability is improved. On the other hand, the welding workability is based on the viewpoints of arc stability, spatter generation amount, slag peeling property, bead affinity, bead appearance and the like.

The coated arc welding electrode according to the present invention is characterized in that the total amount of C: 0.05 to 0.10 mass%, Si: 1.0 to 1.3 mass%, Mn : 0.8 to 1.2 mass%, Cr: 1.3 to 1.9 mass%, Mo: 0.5 to 1.0 mass%, V: 0.3 to 0.5 mass%, Nb: 0.02 to 0.06 mass%, B: 0.005 to 0.015 mass%, Mg: 0.05 0.10 mass% or less of Cu, Ni, Ti, Al, Ta, Co, and W as the inevitable impurities, and S, Sn, As, Sb, Pb, Bi is preferably 0.10% by mass or less.

According to such a constitution, by further restricting the range of the component contained in either or both of the core wire and the coating of the coated arc welding electrode, the creep rupture performance, toughness, tempering brittleness, Is more likely to be improved.

According to the coated arc welding electrode of the present invention, it is possible to obtain a weld metal excellent in creep rupture performance, toughness, tempering embrittlement resistance and SR resistance resistance. Further, according to the coated arc welding electrode of the present invention, the welding workability is excellent.

FIG. 1A is a schematic view showing an improvement (opening) shape of a welding test body used in an embodiment of the present invention. FIG.
1B is a schematic view showing an improved shape of a welding test body used in an embodiment of the present invention.
2 is a graph showing the PWHT condition in the embodiment of the present invention.
3 is a graph for explaining the step cooling processing conditions in the embodiment of the present invention.
4A is a schematic view for explaining a test piece used in the evaluation of the resistance to SR fracture in the embodiment of the present invention.
Fig. 4B is a schematic view for explaining a test piece used in the evaluation of the resistance to SR fracture in the embodiment of the present invention. Fig.
4C is a schematic diagram for explaining a test piece used in the evaluation of the resistance to SR fracture in the embodiment of the present invention.
Fig. 4D is a schematic view for explaining a test piece used in the evaluation of the resistance to SR fracture in the embodiment of the present invention. Fig.
5 is a graph showing the SR condition of evaluation of the internal SR cracking property in the embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail.

The coated arc welding electrode of the present invention is characterized in that, during the coating operation, the metal carbonate in an amount of 5 to 10 mass% in terms of CO ₂ , the alkali metal oxide in an amount of 0.4 to 2.0 mass% in terms of alkali metal, In terms of F is 1.0 to 5.0 mass%.

In addition, the coated arc welding electrode can be manufactured by adding one or both of the core wire and the coated wire of the coated arc welding electrode and the total amount of C, Si, Mn, Cr, Mo, V, Nb, B, Mg, Fe And the total sum of Cu, Ni, Ti, Al, Ta, Co and W is not more than a predetermined amount and the total sum of S, Sn, As, Sb, Pb and Bi is made to be not more than a predetermined amount as inevitable impurities .

Hereinafter, each configuration will be described.

[Including metal carbonate, alkali metal oxide, fluorine compound, during coating, per covered mass of arc electrode]

≪ Metal carbonate: 5 to 10 mass% in terms of CO ₂ >

The metal carbonate is decomposed into metal oxides and CO ₂ by arc heat. The former forms slag to protect the weld bead, and the latter is added during coating to reduce the hydrogen partial pressure in the arc atmosphere and reduce the amount of diffusible hydrogen in the weld metal. The metal carbonate is not particularly limited, such as MgCO ₃ , CaCO ₃ , BaCO ₃ , and Na ₂ CO ₃ . However, if the content is too small, the amount of slag generated is insufficient to deteriorate the appearance of beads, and the amount of diffusible hydrogen in the deposited metal increases, thereby increasing the susceptibility to cold cracking. Particularly, the increase of the amount of diffusible hydrogen in the weld metal is an extremely serious problem in the case of high strength steel such as high strength 2.25Cr-1Mo-V steel. On the other hand, if these metal carbonates are added in excess, the viscosity of the molten slag increases, and the molten slag becomes prone to precede, so that the affinity of the weld bead deteriorates, causing welding defects such as slag inclusion. Therefore, the content of the metal carbonate per total mass of the coated arc welding electrode is 5 to 10 mass% in terms of CO ₂ conversion.

<Alkali metal oxide: 0.4 to 2.0 mass% in terms of alkali metal>

The alkali metal oxide serves as a binder for forming a coating, and is contained in water glass added during coating to adhere to the core wire. Examples of the chemical composition include Na ₂ O, K ₂ O and Li ₂ O, but these alkali metal oxides also affect welding workability of the coated arc welding electrode. Even if these alkali metal oxides are excessively large, at least too much, that is, if the addition amount of water glass is excessively large, the welding workability, particularly arc stability, is adversely affected. Therefore, the content per mass of the coated arc welding electrode is set to 0.4 to 2.0% by mass in terms of alkali metal.

&Lt; Fluorine compound: 1.0 to 5.0 mass% in terms of F value >

The fluorine compound is added to the coating agent as a slag forming agent. The fluorine compound is not or BaF _2, CaF _2, KF, NaF, AlF _3, etc. as well as metal fluorides typified by, K ₂ SiF ₆ or complex metal fluorides typified by PTFE and high molecular polymers, particularly limited. However, if the content thereof is excessively small or excessively large, the bead shape is deteriorated. Therefore, the content per total mass of the coated arc welding electrode is 1.0 to 5.0 mass% in terms of F value.

[Including one or both of the core wire and the covering of the covered arc welding electrode, including a predetermined amount of the predetermined element per the total mass of the covered arc welding electrode]

&Lt; C: 0.04 to 0.15 mass%

C has a great influence on the hardenability of the deposited metal and the precipitation form of the carbide. If the amount of C in the deposited metal is low, since the bainite transformation temperature becomes high, the bainite structure becomes coarse, and toughness and tempering characteristics are deteriorated. Further, the deposition amount of the carbide becomes insufficient, and the creep rupture strength also decreases. On the other hand, if the amount of C is excessively high, the precipitation amount of the carbide increases, the creep strength increases, the coarsening of the carbide precipitated is accelerated, the toughness and tempering characteristics are deteriorated, SR Cracks and other various types of cracks. Further, the arc stability is deteriorated. Therefore, the amount of C per total mass of the coated arc welding electrode is 0.04 to 0.15 mass%. Preferably, the lower limit is 0.05% by mass and the upper limit is 0.10% by mass.

&Lt; Si: 1.0 to 1.5 mass%

Si has the effect of securing the room temperature strength of the deposited metal and deoxidizing and cleaning the deposited metal to improve the toughness. If the amount of Si in the deposited metal is too low, the creep strength is lowered, and furthermore, the amount of oxygen in the deposited metal is increased to form a coarse oxide, thereby lowering the toughness. Further, bead affinity and bead appearance deteriorate. On the other hand, excessive addition of Si deteriorates the tempering embrittlement property. Therefore, the amount of Si per total mass of the coated arc welding electrode is set to 1.0 to 1.5% by mass. Preferably, the lower limit is 1.0% by mass and the upper limit is 1.3% by mass.

&Lt; Mn: 0.7 to 1.2 mass%

Since Mn is an element lowering the bainite transformation temperature of the deposited metal subsequent to C, Mn is added to decrease the bainite transformation point to form a microstructure and improve the toughness. On the other hand, there is a contradictory effect that the creep strength is lowered by the increase of the sliding site accompanying the miniaturization of the microstructure. Further, Mn, like Si, deoxidizes and cleanses the deposited metal to improve its toughness, while excessive addition deteriorates the tempering characteristics. Therefore, the amount of Mn per total mass of the coated arc welding electrode is set to 0.7 to 1.2% by mass from the viewpoint of both the toughness and tempering characteristics of the deposited metal and the creep rupture strength. Preferably, the lower limit is 0.8% by mass and the upper limit is 1.2% by mass.

&Lt; Cr: 1.3 to 1.9 mass%

Cr, together with Mo and V, is a basic component of high strength 2.25Cr-1Mo-V steel. Cr is an indispensable component for improving the room temperature strength and creep strength and for improving the corrosion resistance by preferentially forming a carbide in the spherical grain boundary. According to the study by the present inventors, it has become clear that the lowering of Cr accelerates precipitation of fine carbides, and the increase of Cr accelerates precipitation of coarse carbides. If the amount of Cr in the deposited metal is excessively small, fine carbides are excessively precipitated to not only lower the toughness but also deteriorate the SR cracking resistance. On the other hand, when the amount of Cr is increased, the precipitated carbides are coarsened to lower not only the toughness and the creep strength but also promote intergranular segregation of the impurities, deteriorating the tempering embrittlement property and the SR SR resistance. Therefore, the amount of Cr per total mass of the coated arc welding electrode is set to 1.3 to 1.9 mass%.

&Lt; Mo: 0.5 to 1.0 mass%

Mo is a basic component of high strength 2.25Cr-1Mo-V steel together with Cr and V. Mo forms carbide and is dissolved in the mother phase to improve its room temperature strength and creep rupture strength. If the amount of Mo in the deposited metal is too small, the deposition amount of carbide and the high solubility in the mother phase are insufficient to lower the creep rupture strength. On the other hand, when the amount of Mo is large, the amount of carbonized material and the amount of high-solids in the casting phase excessively increase, and the strength is remarkably increased to deteriorate toughness. Therefore, the amount of Mo per total mass of the coated arc welding electrode is set to 0.5 to 1.0 mass%.

&Lt; V: 0.3 to 0.5 mass%

V is a basic component of high-strength 2.25Cr-1Mo-V steel together with Cr and Mo. V inhibits the creep diffusion of dislocations by forming fine carbides, thereby improving room temperature strength and creep rupture strength. If the amount of V in the deposited metal is too small, the deposition amount of the carbide is insufficient and the creep rupture strength is lowered. On the other hand, if the amount of V is increased, the precipitation amount of carbide excessively increases, and the toughness, the tempering embrittlement property and the SR SR resistance are lowered. Further, the arc stability, the amount of generated spatters, and the appearance of beads deteriorate. Therefore, the V amount per total mass of the coated arc welding electrode is set to 0.3 to 0.5 mass%.

&Lt; Nb: 0.02 to 0.06 mass%

Nb, like V, has a strong tendency to form fine carbides, thereby improving the creep strength. If the amount of Nb in the deposited metal is excessively small, the deposition amount of carbide is insufficient and the creep strength is lowered. On the other hand, if the amount of Nb is increased, the precipitation amount of the carbide excessively increases, and the toughness, the tempering embrittlement property and the SR SR resistance are lowered. Also, the amount of spatter generated increases. Therefore, the amount of Nb per total mass of the coated arc welding electrode is 0.02 to 0.06 mass%.

&Lt; B: 0.005 to 0.015 mass%

B has an effect of enhancing the hardenability of the deposited metal by adding a small amount of the metal, improving the toughness of the structure and improving the creep strength. If the amount of B in the deposited metal is too small, the structure becomes coarse, toughness is lowered, and the creep rupture strength is lowered. On the other hand, if the amount of B is increased, the SR cracking resistance is remarkably lowered. Therefore, the amount of B per total mass of the coated arc welding electrode is 0.005 to 0.015 mass%.

&Lt; Mg: 0.05 to 0.15 mass%

Mg deoxidizes and cleanses the deposited metal to improve toughness and improves arc stability and re-arcability at the time of welding. If the amount of Mg in the deposited metal is excessively small, the amount of oxygen in the deposited metal increases to form a coarse oxide, which deteriorates toughness, and also causes poor arc stability and spatter generation. On the other hand, even if the amount of Mg is increased, the arc stability, the amount of generated spatters and the appearance of beads become poor. Therefore, the amount of Mg per total mass of the coated arc welding electrode is set to 0.05 to 0.15 mass%. Preferably, the lower limit is 0.05% by mass and the upper limit is 0.10% by mass.

&Lt; Fe: 60 to 75 mass%

Fe is a main component constituting the deposited metal, and is mainly added from the core wire of the coated arc welding electrode, but may be added from the coating agent. Particularly, it is possible to increase the deposition efficiency by adding iron powder or alloy iron from the coating agent. If the addition amount is excessively large or at least excessively large, the effect of addition of other alloy components can not be sufficiently obtained. Therefore, the amount of Fe per total mass of the coated arc welding electrode is set to 60 to 75 mass%. If the addition amount of Fe is less than 60 mass%, the slag peeling property and the bead appearance deteriorate.

<Inevitable impurities>

In the present invention, the total amount of Cu, Ni, Ti, Al, Ta, Co and W is 0.10 mass% or less and the total sum of S, Sn, As, Sb, Pb and Bi is 0.10 mass% or less as unavoidable impurities.

Cu, Ni, Ti, Al, Ta, Co, and W contribute to the strengthening of the deposited metal as the solid solution strengthening element or the carbonitride forming element. However, due to the design of the welding material in the present application, these elements are present in the deposited metal as unavoidable impurities, which hinders the addition effect of the main alloy component. Therefore, the total amount thereof is 0.10 mass% or less. On the other hand, the lower limit value is not specifically defined, but is, for example, 0.03 mass% as a measurement limit.

When the total amount of Cu, Ni, Ti, Al, Ta, Co, and W exceeds 0.10 mass%, the spatter generation amount increases. Furthermore, when the content exceeds 1.00 mass%, the arc stability also deteriorates.

S, Sn, As, Sb, Pb and Bi are present as inevitable impurities in the deposited metal to lower the grain boundary strength and deteriorate the tempering embrittlement characteristics and SR resistance resistance. Therefore, the total amount thereof is 0.10 mass% or less. On the other hand, the lower limit value is not specifically defined, but is, for example, 0.01% by mass as a measurement limit.

Example

Hereinafter, in order to explain the effects of the present invention, the examples falling within the scope of the present invention and the comparative examples deviating from the scope of the present invention are compared and explained.

[Chemical composition of coated arc electrode]

The coated arc electrode is composed of a solid core wire and a coating agent, and the mass (= coating ratio) of the coating agent is set to 25 to 40 mass% with respect to the total mass of the coated arc electrode. The chemical composition of the coated arc electrode is shown in Tables 1 and 2. On the other hand, numerical values are indicated by underlining those not satisfying the scope of the present invention.

[Welding conditions]

FIG. 1A shows an improved shape of the welding test body used in the embodiment of the present invention. The trial version is ASTM A387. 2.25Cr-1Mo steel such as Gr.22 steel or carbon steel such as JIS G3106 SM490A steel in which the improvement surface is subjected to buttering welding about two to three layers with a blank as shown in Fig. 1B may be used. Since the test results are the same regardless of which trial version is used, ASTM A387 Gr.22 Cl.2 steel, which is a publicly funded system, is used in the embodiment of the present invention. The plate thickness of the test plate and backing plate was 20 mm, the improved shape was improved by 20 ° V, and the root gap was 19 mm. The welding length was 300 mm to 600 mm, and the welding was carried out in two passes of one layer, and the number of layers was 8 layers.

Table 3 shows the welding conditions at the time of the welding test. AC or DCEP is the common polarity of the power supply when welding with a covered arc welding rod. However, in the welding test in this application, the performance of the deposited metal was the same regardless of which power polarity was used. Therefore, in all of the examples, the test results by the AC power source are described. In addition, core wire diameters of the coated arc welding electrodes for Cr-Mo steel are generally 2.6, 3.2, 4.0, 5.0 and 6.0 mm ?. However, in the welding test in this application, the performance of the deposited metal was the same regardless of which core wire electrode of the core wire diameter was used. Therefore, in all of the examples, the test results of the coated electrode wire electrode of core wire diameter of 5.0 mm? In addition, the welding test was carried out with the welding posture downward.

[PWHT]

Fig. 2 shows various PWHT conditions applied to the welding test material. The strength of the PWHT can be quantified by introducing the concept of the heat treatment parameter of Larson-Miller (Larson Miller) (abbreviated as [P]). [P] is expressed by Equation 1 below, where T is the PWHT temperature (° C), and t is the PWHT time (h).

That is, the higher the PWHT temperature and the longer the PWHT time, the larger the [P], and the deposited metal is further annealed. Particularly, the cohesion and coarsening of the carbonitride, which plays a large role in securing the strength of the low-alloy heat-resisting steel represented by Cr-Mo steel, proceeds, so that the strength is lowered. On the other hand, [P] increases and the strength of the deposited metal decreases, so that the toughness is improved in inverse proportion. However, if the coagulation and coarsening of the carbonitride proceeds excessively, these coarse carbonitrides act as a starting point of brittle fracture, and in some cases, the toughness deteriorates. Further, the tendency of increase and decrease in strength and toughness is almost linear with respect to [P], and high-temperature strength represented by creep rupture strength also shows a similar tendency.

(1) 705 (± 15) ° C. × 8 (± 1) h and (2) 705 (± 15) ° C. × 32 (1) 20.07 to 20.81, and (2) 20.70 to 21.37, respectively.

In evaluating the temper embrittlement characteristics of the deposited metal, a test material subjected to special embrittlement-promoting heat treatment called step cooling (step cooling (SC), FIG. 3) described later was used.

From here,

Min.PWHT (703 占 폚 占 8h): Charpy impact test (condition of Fig. 2 (a))

Min.PWHT (703 DEG C x 8h) + S.C. Charpy impact test (for evaluating tempering embrittlement characteristics)

Max.PWHT (708 占 폚 x 32h): creep rupture test (condition of Fig. 2 (b)

to be.

<Step cooling>

Next, step cooling will be described. Fig. 3 shows a graph in which the ordinate axis indicates the temperature and the abscissa axis indicates time, for explaining the process conditions of step cooling. As shown in Fig. 3, in the step cooling, the sealing material is heated. When the temperature of the sealing material exceeds 300 deg. C, the heating conditions are adjusted so that the temperature increasing rate becomes 50 deg. C / h or less so that the temperature of the sealing material reaches 593 deg. I heated until I did. After keeping at 593 캜 for one hour, the steel material was cooled to a temperature of 538 캜 at a cooling rate of 6 캜 / h and maintained for 15 hours, cooled to 523 캜 at the same cooling rate, held for 24 hours, The mixture was cooled to 495 DEG C and maintained for 60 hours. Next, the test piece was cooled to 468 占 폚 at a cooling rate of 3 占 폚 / hour and maintained for 100 hours. Then, the specimen was cooled so that the cooling rate was 28 ° C / h or less until the temperature of the specimen became 300 ° C or less. On the other hand, in this process, the temperature raising rate and the cooling rate are not specified in the temperature range where the temperature of the specimen is 300 ° C or lower.

[Toughness and tempering properties (Charpy impact test)]

In evaluating the toughness and tempering characteristics of the weld metal, a Charpy impact test specimen of 2 mm-V notch of ISO 148-1 was taken from both test materials with PWHT conditions of 703 ℃ × 8h and 703 ℃ × 8h + step cooling. , And three Charpy impact tests were carried out at three temperatures of 20, 0, -18, -30, -50, -70 and -90 캜. Next, the Charge Transition Temperature (hereinafter referred to as vTr54 and vTr'54) indicative of 54J is determined from the mutual transition curve passing through the test value at each temperature by lubrication, and the variation amount of vTr54 (= vTr54 '54 -vTr54, hereinafter referred to as? VTr54) was calculated as an embrittled amount.

The temper embrittlement susceptibility of the weld metal is judged synthetically from both of vTr54 and? VTr54. Here, a value (= vTr54 + 3? VTr54) obtained by adding a value obtained by tripling? VTr54 to vTr54 was defined as a tempering brittle characteristic value. Considering that the step cooling is an accelerated test and the practical working life is several tens of years, it is considered that the actual deposited metal is caused to embrittle three times as much as the embrittlement amount due to step cooling, and the deposited metal after long- (3 in this case) of ΔvTr54 tends to become stricter every year from 1.5 → 2.0 → 2.5 → 3.0 as in the creep performance, and the coefficient 3 It is the most strict in the demands of the near real. Even when the embrittling amount (=? VTr54) is small, when vTr54 is large, vTr54 + 3? VTr54 is also large, so that it is not preferable as the application point of the weld metal in which stable welding quality is required. On the other hand, even when the toughness after PWHT (= vTr54) is excellent, when the embrittlement amount (=? VTR54) is large, vTr54 + 3? VTr54 also becomes large and is not preferable for the same reason.

In the present example, the toughness was evaluated as?, A three point average of the impact value at -30 占 폚 (hereinafter referred to as vE-30 占 폚) of 100 J or more,?

With respect to the embrittlement amount,? DELTA vTr54 of 5 占 폚 or less was rated as?, 5 占 폚 and less than 11 占 폚 was rated?, And 11 占 폚 or more was evaluated as 占.

With respect to the tempering embrittlement property values,?,? TT54 + 3? VTr54 having a temperature of -30 占 폚 or less was rated as?, -30 占 폚 or less and 0 占 폚 or less and?

From here,

vTr54: Charge transition temperature (° C) representing 54J after PWHT

vTr '54: Charge transition temperature (° C) representing PWJ + 54J after step cooling

? VTr54 (= vTr'54-vTr54): Embrittlement amount by step cooling (占 폚)

vTr54 + 3? vTr54: tempering embrittlement characteristic (占 폚)

to be.

[Creep rupture test]

Creep rupture test specimens of ISO 204 conforming to the PWHT condition of 708 占 폚 32 hours were taken and subjected to a creep rupture test at a test temperature of 540 占 폚 and a load stress of 210 MPa. In the present embodiment, the breaking time (hereinafter referred to as &quot; Tr &quot;) was 1500 h or more.

[Evaluation test for internal SR cracking (ring crack test)]

SR (stress relief annealing) is often applied for the main purpose of reducing the residual stress of the structure during the manufacturing process of the PWHT prior to the application of the PWHT in the manufacture of the low-alloy heat resistant forced pressure vessel including Cr-Mo steel. It is the SR crack that arises at the old grain boundary at this SR. The SR crack has an excessive difference in strength between the crystal grain boundary and the old? Grain boundary surface due to the deposition of carbide in the crystal grains due to SR and the segregation of impurities in the? -Grain boundary, resulting in a relatively weakened sphere γ grain boundary can not resist the residual stress.

In the evaluation of the SR fracture resistance of the weld metal in the present application, a test method called a ring crack test was applied. A ring test piece having a U-groove (U notch) and a slit (slit width: 0.3 mm) was taken from the welded test piece as shown in FIG. 4A . The number of tests is two. The shape of the test piece is shown in Fig. 4B.

As shown in Fig. 4C, the slits were TIG-welded to a thickness of about 0.05 mm, and the residual stress was imposed directly under the U-grooves. Next, SR (625 占 폚 占 10h) under the conditions shown in Fig. 5 was applied to the test piece after TIG welding, and then the test piece was divided into three parts in the longitudinal direction as shown in Fig. 4d, The presence or absence of SR generation at the? -Grain boundary was observed. The results are shown in Table 1. The results are shown in Table 1. The results are shown in Table 1. The results are shown in Table 1. The results are shown in Table 1. The results are shown in Table 1. [

[Welding workability]

The welding workability when the welding test was conducted under the welding conditions shown in Table 2 was evaluated sensibly. First, with respect to "arc stability", it was evaluated that a weak fluttering tendency of the arc was ◎, a slightly weaker one was ○, and a stronger one was ×. Secondly, the spatter generation tendency was evaluated as "poor", "slightly spatter generated", and "strong spatter generation tendency" as "x". Third, the slag peeling property was evaluated as "easy peeling", "relatively easy" and "poor" as "x". Fourthly, ⊚ indicates that the slag inclusion tendency toward the bead end is extremely weak, ◯ indicates that the slag inclination tendency is weak, and X indicates the strongest. Fifth, with respect to the "bead appearance", it was evaluated that the waves of the beads were extremely smooth and smooth, that they were smooth, that they were smooth, and those that were rough and uneven.

The results are shown in Table 4.

As shown in Table 4, 1 to 10 satisfy the range of the present invention, good results were obtained in all evaluation items.

On the other hand, 11 to 40 do not satisfy the range of the present invention, the following results are obtained.

No. 11 to 16 were inferior in welding workability because either the CO ₂ conversion value of the metal carbonate, the alkali metal oxide value of the alkali metal oxide value or the F value of the fluorine compound was out of the range of the present invention. No. 17, the C content was less than the lower limit value, evaluation of toughness, embrittlement amount, tempering embrittlement property and creep rupture performance was bad. No. 18, the content of C exceeded the upper limit value, so evaluation of toughness, amount of embrittlement, tempering embrittlement property, resistance to SR cracking, and weldability were bad.

No. 19 had an Si content lower than the lower limit value, evaluation of toughness, brittleness, creep rupture performance, and welding workability was bad. No. 20 had an Si content exceeding the upper limit value, evaluation of the embrittled amount and the tempering embrittlement property was bad. No. 21 had poor Mn content because the Mn content was less than the lower limit value. No. 22 had a Mn content exceeding the upper limit value, evaluation of the embrittlement amount, tempering property and creep rupture performance was bad. No. Since the Cr content is less than the lower limit value, evaluation of toughness and resistance to SR cracking is poor. No. 24 had a bad Cr content, exceeding the upper limit value, evaluation of toughness, embrittling amount, tempering embrittlement property, creep rupture performance, and SR resistance resistance.

No. 25 had an Mo content lower than the lower limit value, evaluation of the creep rupture performance was bad. No. 26, the Mo content exceeded the upper limit value, and thus the evaluation of toughness was bad. No. 27 had a V content lower than the lower limit value, the evaluation of the creep rupture performance was bad. No. 28, the V content exceeded the upper limit, so evaluation of toughness, tempering embrittlement resistance, resistance to SR cracking, and welding workability was bad. No. 29, the Nb content was less than the lower limit value, and thus evaluation of the creep rupture performance was bad. No. 30, the Nb content exceeded the upper limit value, so evaluation of the toughness, the amount of embrittlement, the tempering embrittlement property, the resistance to SR cracking, and the workability of welding were bad.

No. 31, B content was less than the lower limit value, evaluation of toughness and creep rupture performance was bad. No. 32, the content of B exceeded the upper limit value, and hence the evaluation of the resistance to SR cracking was bad. No. 33 had an Mg content lower than the lower limit value, and therefore, evaluation of toughness and welding workability was bad. No. 34 had a Mg content exceeding the upper limit value, and therefore evaluation of welding workability was bad. No. 35 had an Fe content lower than the lower limit value, evaluation of toughness, tempering embrittlement property, creep rupture performance, and welding workability was bad. No. 36, the Fe content exceeded the upper limit value, so that evaluation of toughness, amount of embrittlement, tempering property and creep rupture performance were bad.

No. 37, the total of Cu, Ni, Ti, Al, Ta, Co, and W exceeded the upper limit value, and evaluation of toughness, creep rupture performance, and welding workability was bad. No. 38, the total of Cu, Ni, Ti, Al, Ta, Co, and W exceeded the upper limit value, so evaluation of toughness, embrittlement amount, tempering embrittlement property, creep rupture performance, resistance to SR cracking and weldability was bad. No. 39, the sum of S, Sn, As, Sb, Pb, and Bi exceeded the upper limit value, and evaluation of the embrittlement amount, tempering embrittlement property, and SR SR resistance was bad. No. 40, the total sum of S, Sn, As, Sb, Pb, and Bi exceeded the upper limit value, and evaluation of the embrittlement amount, tempering embrittlement property, and SR resistance resistance was bad.

Although the present invention has been described in detail with reference to the embodiments and the examples, the spirit of the present invention is not limited to the above description, and the scope of the right should be broadly interpreted based on the description of the claims. It is needless to say that the contents of the present invention can be broadly modified or changed on the basis of the above description.

Claims (2)

Wherein the amount of the metal carbonate in terms of CO ₂ is from 5 to 10 mass%, the alkali metal oxide in terms of alkali metal is from 0.4 to 2.0 mass%, the fluorine compound is from 1.0 to 5.0 mass% % By mass,
C: 0.04-0.15% by mass, Si: 1.0-1.5% by mass, Mn: 0.7-1.2% by mass, and Cr: Wherein the alloy contains at least one element selected from the group consisting of Ba: 1.3 to 1.9 mass%, Mo: 0.5 to 1.0 mass%, V: 0.3 to 0.5 mass%, Nb: 0.02 to 0.06 mass%, B: 0.005 to 0.015 mass%, Mg: 0.05 to 0.15 mass% And the total amount of S, Sn, As, Sb, Pb and Bi is 0.10% by mass or less as the inevitable impurities, the total content of Cu, Ni, Ti, Al, Ta, Co and W is 0.10% Features a covered arc welding electrode. The method according to claim 1,
C: 0.05 to 0.10% by mass, Si: 1.0 to 1.3% by mass, Mn: 0.8 to 1.2% by mass, and Cr : 0.3 to 1.9 mass%, Mo: 0.5 to 1.0 mass%, V: 0.3 to 0.5 mass%, Nb: 0.02 to 0.06 mass%, B: 0.005 to 0.015 mass%, Mg: And the sum total of S, Sn, As, Sb, Pb and Bi is 0.10% by mass or less as the inevitable impurities, the total content of Cu, Ni, Ti, Al, Ta, Co and W is 0.10% Wherein the coated arc electrode comprises at least one electrode.

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